Decoding Deworming

Decoding Deworming

July 13, 2018

In recent years, it has become apparent that our approach to treating internal parasites must evolve, largely due to the growing problem of resistance to deworming drugs. The traditional line of attack developed in the 1960s was to treat horses with a different category of dewormer every 4-6 weeks and this was based on what we knew at the time about the life cycles of common intestinal parasites. Although straightforward, this method is costly, creates problems with anthelmintic resistance and basically lacks understanding of the equine immune system and the real effect worms have on the horse. Essentially the practice of over treating clinically insignificant intestinal parasites allows the worms to evolve to become resistant to anthelmintic drugs.


The modern approach to gastrointestinal parasites aims to reduce overall treatments by putting the emphasis on diagnosis (testing faecal samples for parasite eggs known as a faecal egg count) and management practices. Worm eggs are shed very unequally among a herd of horses such that 20% of the horses shed 80% of the eggs. The first step in this approach is to identify these high shedders for treatment to reduce environmental contamination and protect vulnerable horses. We recommend treating horses with strongyle eggs per gram greater than 200 or if the horse has any Parascaris equorum, Strongylus or fluke eggs.


Formulating the right plan: It should be noted that there is not a one size fits all parasite control strategy. We should consider the specific needs of the population of horses on the farm and how they are managed throughout the year. For most mature horses, we advocate twice-yearly deworming: once in the spring against strongyles and again in the autumn against strongyles and tapeworms and also including pre- and post-treatment faecal egg counts (FECRTs). Monitoring drug effectiveness is essential for maintaining herd health and protecting the overall equine population.


It is important to know which drugs you are using and not just the brand name. Unfortunately, there is no single drug to treat all parasite categories.

Benzimidazoles This class includes fenbendazole (Panacur) and oxibendazole (Anthelcide EQ) and have been around for decades. They continue to have good efficacy against ascarids but small strongyles consistently show resistance. Their use is often limited to foals these days. Fenbendazole is a safe drug and is often given as a double dose on five consecutive days.

Pyrimidines include the drug pyrantel known by the trade name Strongid. Pyrantel does not penetrate the intestinal wall and, so, will not kill encysted strongyles or any parasite stage outside the intestine. There is now significant resistance to pyrantel among strongyles anyway. Pyrantel also comes in pelleted form that can be added to feed as a continuous dewormer (Strongid-C). Double-dose pyrantel is labelled for tapeworm treatment and is effective. It is generally very safe for all ages of horses.

Macrocyclic lactones (ivermectin and moxidectin) Ivermectin has been our most relied-upon dewormer, but certain parasites are developing resistance to it. Unlike the other drug classes, macrocyclic lactones also kill external parasites such as lice, mites, ticks and bot larvae found in the stomach. Moxidectin can penetrate the intestinal wall and kill encysted strongyles, and it probably is the most effective compound for this purpose.

Praziquantel This drug kills only tapeworms and is currently marketed in combination with either ivermectin or moxidectin. Praziquantel is probably also being overused, especially in regions that have very few tapeworms i.e. hot dry climates. To assess the efficacy of these drugs, we use a faecal egg count reduction test (FECRT). This is basically a pretreatment egg count test followed by a second FEC 10-14 days later which would show at least a 90% reduction in egg counts. Anything less than this means there is resistance among those parasites.

While there are great differences among the important internal parasites, they share a fundamentally similar life cycle- Adult worms live, reproduce and lay their eggs in the horse’s intestine which are passed with faeces into the environment. The eggs hatch in the pasture releasing larvae that develop to a point at which they are “infective” and a horse can inadvertently consume them, where the infective larvae now have an opportunity to develop into adults within the horse and returning us to the start of the cycle. Anthelmintic drugs are generally most effective against adult worms and we do not target the larvae living in the environment with any drugs or chemicals as this would be detrimental to the ecosystem.

  • Ascarids (Parascaris equorum) Foals up to 6 months are particularly susceptible to this large white worm. Because ascarid eggs can live for long periods in the environment, pastures grazed by foals in previous years pose a risk to foals grazing the same pasture this year. Once ingested the larvae can migrate through the foal’s liver and lungs before maturing to adults in the intestine. This process takes about 10 weeks. Oxibendazole or fenbendazole work well against these worms.

  • Small strongyles (cyathostomins) Their larvae burrow into the intestinal wall and stay there for extended periods in a dormant state until the environment is optimal for reproduction, at which time they emerge. This protects them from most of our deworming compounds, which cannot penetrate the intestinal wall well enough to kill the encysted larvae there. Small strongyles are now resistant to many of our common dewormers. Fortunately, these parasites do not cause severe disease unless they are present in extremely large numbers. Signs of clinical cyathostomiasis include weight loss, lack of appetite, diarrhoea, fever, lethargy and dull coat.

  • Tapeworms (Anoplocephala perfoliata) These parasites are common in horses, depending on geographic location, and can cause spasmodic colic and impaction. More commonly found in areas with lush green pasture i.e. conditions that favour its intermediate host, the orbatid mite. Tapeworms are difficult to diagnose. There has not been reported resistance of tapeworms to praziquantel and pyrantel because there are no methods to measure resistance.

  • Large strongyles These are now pretty rare because all dewormer classes are effective against them. Previously these worms were more significant as their migrating larvae could cause blockage of the arteries supplying blood to the intestines resulting in colic.

  • Pinworms (Oxyuris equi). Although they have little serious effects on a horse, they can be a nuisance to get rid of due to fact the pinworm has a particularly long life cycle, taking about 4 months to mature and hence be susceptible to anthelmintic drugs. Female pinworms live in the rectum and emerge to lay their eggs on the skin around the anus which cause irritation and itching. Signs of pinworm infestation include tail rubbing (which spreads the eggs into the environment) and sometimes a yellowish grey substance around the anus. Pinworm infestations can be overcome with one or two monthly treatments and frequently washing the perineum.


  • Thankfully intestinal parasites are not a common cause of death but owners often fret unduly about their presence in beloved horses. Naturally there is a nauseating aspect to overcome but in reality, adult horses can easily deal with a small number of worms and their presence is just the reality of grazing horses. The parasites aim is to live symbiotically with the horse, not to kill its host. We cannot eliminate worm burdens entirely and we might not want to. An immune system that is stimulated by the worms the horse carries is actually beneficial and will improve the immune system. This is also called hormesis.

  • Parasite transmission centres around grazing. Infection pressure increases through the summer as horses are grazing, ingesting worms, and shedding eggs so by early autumn worm burdens are at their peak. A pasture left to rest for 2-3 weeks in hot, dry conditions should have its parasite numbers drop off significantly but might not apply to unpredictable Victorian weather.

  • Manure removal from paddocks is a very effective parasite control practice, especially if stocking rates are high. Most parasites take a week or more to hatch and develop into infective larvae so paddock picking once or twice a week can reduce transmission and ultimately cut down on treatments.

  • Horses that are stalled a lot such as racehorses or injured horses are exposed to fewer pasture-borne parasites. For adult horses under such circumstances we recommend two treatments per year schedule and letting the parasite egg counts determine whether your horses need additional treatments.

  • Immune system suppression associated with transport can cause an increase in egg shedding, not to be confused with an increase in overall worm numbers. This is especially true in young horses and needs to be kept in mind when travelling horses over long distances. A treatment two weeks before travel can alleviate this but all proper protocols for managing new horses should be followed regardless.

  • Newly arrived horses are very unlikely to introduce parasite species that are not already present on a property. However, the horse may be carrying worms that are resistant to the wormer that is working effectively on your property. Quarantine any incoming horses to prevent their parasites (or any infectious diseases) from affecting any of the horses currently on the property. Then treat the horse with a dewormer and perform pre- and post-treatment egg counts to determine if the drug is effective. If you find the horse is harbouring parasites resistant to this drug, repeat this process using a different dewormer.

  • A study in 2011 evaluating parasite load in Standardbred racehorses found that the horses that won the most races actually had the highest egg counts. A possible reason for this is the ongoing stress under which high performing horses are constantly under such as the repeated travel and the physical efforts of frequent racing and a high intensity training. This stress can cause immune system suppression which the worms take advantage of by producing more eggs. Additional treatments may be warranted for these horses.

  • As for young horses, it may be convenient to implement a strategy based on known risk. However, all foals should have an FEC carried out at least once as part of this plan. It is recommended to deworm for ascarids when a foal is 2-3 months old as this is how long it takes for the roundworms to mature and only the mature worms are vulnerable to anthelmintics. By 5 or 6 months or, when most foals are weaned, previously unaddressed ascarid burdens may become a problem. Strongyles are also becoming significant at this stage so this is a good milestone in the foal’s life to perform an FEC, identify which worms are present in the foal and treat accordingly. Thereafter, strongyles are the main concern but do not forget about tapeworm. A combination treatment against strongyles and tapeworm can be given at 8-10 months followed by a fourth deworming, targeting strongyles, when the horse is a year old.

  • The mere presence of parasite eggs or larvae in the faeces does necessarily mean there is parasitic disease. A negative faecal egg count does not necessarily mean the horse is worm-free, just that the adult worms are not laying eggs at that time. Similarly, a positive faecal egg count doesn’t mean that a parasite infection is the reason for an illness but can be coincidental.

As always, if you have any queries don't hesitate to get in touch with us:

Dr Eoin Kelly 0418 978 558